ci: --fetch-depth

Backtracking on not doing this earlier - cml ci --fetch-depth=<int> is a good idea

• depth=0 could be an alias for --unshallow
• could thus deprecate (hide) --unshallow
• things like depth=2 is useful for e.g. dvc metrics diff HEAD~1 CC @shcheklein @dberenbaum @pmrowla
• cml ci --fetch-depth=2 is more trustworthy than manually running git fetch --depth=2 because the latter may not work as expected (?) if the repo was checked out weirdly (vis actions/checkout@v3 breaking changes)
• cml ci --fetch-depth=2 is cross-CI compatible unlike actions/checkout.with.fetch-depth: 2

[deepyaman]

Hello! I’ve been curious about CML for some time, and Hacktoberfest seems like a good opportunity to dive in. Can I take a stab at this issue?

Based on a quick look, I think this would involve modifying similar code as in https://github.com/iterative/cml/pull/957/files. @casperdcl Just to confirm, based on your description, should the fix deprecate --unshallow at the same time?

[deepyaman]

Sorry for the delay. This is still very much on my list of things to do; got busy, but I hope to have a PR by later this week.